High voltage current transformer



April 968 R. H. MILLER 3,380,009

HIGH VOLTAGE CURRENT TRANSFORMER Filed March 10, 1967 2 Sheets-Sheet 1 INVENTOR.

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ATTORNEY April 23, 1968 R. H. MILLER 3,380,009

HIGH VOLTAGE CURRENT TRANSFORMER Filed March 10, 1967 2 Sheets-Sheet 2 v //v VENTO/P.

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I [I 1' m I W 6766M A TTORNEY United States Patent 3,380,009 HIGH VOLTAGE CURRENT TRANSFORMER Richard H. Miller, Bcrwyn, Pa., assignor to General Electric Company, a corporation of New York Filed Mar. 10, 1967, Ser. No. 622,204 7 Claims. (Cl. 336-92) ABSTRACT OF THE DISCLQSURE A high voltage current transformer comprising an insulating column, a high voltage tank mounted on the insulating column, and low voltage secondary windings mounted on a tubular insulator within the tank. A metal tension member extends through the column and through the tubular insulator. Tensile forces applied to the tension member are transmitted to the insulator and the insulating column as compressive forces, which strengthen the insulating column and improve its ability to withstand high loads.

Background of the invention This invention relates to a high voltage current transformer and, more particularly, relates to a high voltage current transformer that includes an insulating column loaded in compression for electrically isolating the high voltage parts of the transformer from ground.

Most high voltage current transformers include a column of insulating material for electrically isolating the high voltage parts of the transformer from ground. Typically, this insulating column must be designed to withstand large mechanical forces, as might result, for example, from internal pressures, from cable pull, from heavy ice formations, or from high winds. Since many insulating materials are relatively weak in tension, it has been proposed that means he provided for preloading the insulating column in compression to increase its ability to withstand these high loads, which might otherwise prduce high tensile stresses.

Prior arrangement for compressively preloading the insulating column have usually been expensive and have consumed considerable space, which in some cases has necessitated increasing the size of the insulating column to accommodate the preloading means.

Summary An object of the present invention is to construct the current transformer in such a manner that simple and inexpensive means can be used for preloading the insulating column in compression without requiring any substantial increase in the size of the insulating column to accommodate the preloading means.

In carrying out the invention in one form, I provide a high voltage current transformer that comprises an insulating column, a high voltage tank mounted on the insulating column, and one or more secondary windings at ground potential mounted on a tubular insulator within the tank. A metal tension member at ground potential extends through the column and through the tubular insulator. Tensile forces applied to the tension member are transmitted to the insulator and the insulating column as compressive forces, which strengthen the insulating column and improve its ability to withstand high loads. The tension member is a hollow member that serves as a conduit for carrying the leads from the secondary windings to a region adjacent the lower end of the column.

Brief description of drawings For a better understanding of the invention, reference may be had to the following description taken in conjunction with the accompanying drawings, wherein:

3,380,009 Patented Apr. 23, 1968 FIG. 1 is a side elevational view of a current transformer embodying one form of the invention.

FIG. 2 is an enlarged side-elevational view in section of the current transformer of FIG. 1.

Description of preferred embodiment Referring now to the drawing the current transformer shown therein comprises a base 10 upon which is mounted in upright position a tubular column 12 of insulating material, preferably of porcelain. Mounted atop the column 12 is a metallic tank 14 having an opening 16 therein that is in substantial alignment with the interior of the tubular insulating column. The base 10 is normally at ground potential, and the tank 14 is normally at a high voltage. The insulating column 12 serves to electrically isolate the tank 14 from the base 10 and from nearby parts at or near ground potential. The illustrated tank 14 comprises two major parts 14a and 14b joined together by a flanged horizontally-extending joint 17.

The tank 14, the insulating column 12, and the base 10 are clamped together in pressure-tight relationship by a tubular metal tension member 18 preferably of steel. Tension member 18 extends from a point within the tank 14, downwardly through the insulating column -12 along its central longitudinal axis, to a point near the base 10. At its upper end the tension member 18 has an adjustable flange 20 that bears against a metal supporting fixture 22, which, in turn, bears against the top of a tubular insulator 24. Tubular insulator 24 surrounds the tension member 18, is substantially aligned with the insulating column 12, and has its lower base supported on a portion of the metal tank 14. Downwardly acting tensile forces applied to tension member 18 near its lower end (by means soon to be described) act through the parts 18, 20, 22 and insulator 24 to clamp the tank 14, insulating column 12, and base 10 together. Suitable gaskets 26 are positioned between these components 14, 12 and 10 to provide a good pressure-tight seal between them when they are clamped together in this manner.

For applying a downward tensile force to the tension member 18, a series of studs 30 are provided at spaced intervals around the tension member 18 near its lower end. These studs are fixed to the base 10 and have suitable nuts 32 threaded on their upper ends. A flange 34 is welded or otherwise fixed to the tension member 18 at its lower end, and the studs 30 project freely through suitable openings provided therein. Surrounding each stud is a stack of spring, or Belleville washers 36. These spring washers 36 are shown in a compressed condition in which they exert a large downward force on flange 34 and tension member 18.

For adjusting the force derived from spring washers 36, the adjustable flange 20 at the top of the tension member 18 is relied upon. Flange 20, in effect, constitutes a nut threaded onto the upper end of the tension member 18. When this nut 20 is tightened, it effectively shortens the tension member 18, thus compressing the spring washers 36 to adjust the force derived therefrom.

The transformer is assembled by first mounting the tension member 18, without the upper nut 20, on the base 10. The insulating column 12 is not then present. The nuts 31 on the lower studs 30 are then suitably adjusted. Then the column 12 is slipped over the tension member 18; the parts 14b, 24, and 22 are mounted on column 12, after which the nut 20 is threaded onto tension member 18 and suitably tightened to provide the desired compression of springs 36.

The primary conductor of the current transformer is a stud 40, preferably of a highly conductive material such as copper or aluminum. This stud 40 can be connected in series with a high voltage power circuit 41 to carry the current flowing through the power circuit. Stud 40 extends through tank 14 in a horizontal direction and has a flange 42 joined to its left hand end and located outside tank 14. Flange 42 is suitably bolted to the tank, and a suitable gasket 43 is provided between flange 42 and the tank wall to form a pressure-tight seal at this point. There is an electrical connection between tank 14 and flange 42 which is schematically indicated at 44. But aside from this electrical connection in the region of flange 42, the tank 14 is electrically insulated from the primary conductor 40. Electrical connection 44 maintains the tank 14 at the same high voltage as the primary conductor 40.

The right hand end of stud 40 extends through the right hand wall of tank 14 but is locally insulated therefrom. In this regard, there is a nut 45 threaded on the right hand end of stud 40 and bearing against the gasket 46 located between the tank wall and nut 45. This gasket 46 locally insulates the right hand end of the stud 40 from tank 14 and also provides a pressure-tight seal between the tank and the stud at the right hand end of the stud. In the illustrated embodiment, there is a back-up stop 48 fixed to stud 40 just inside tank 14 and another gasket 49 between this stop and tank wall. When the nut 45 is tightened, it compresses gaskets 46 and 49 and clamps the tank wall between stop 48 and the nut 45.

In the illustrated embodiment of the invention, two current transformer secondary windings 50 and 52 are shown within tank 14. It is to be understood, however, that any desired number of such windings could be provided, depending upon the intended application of the transformer. Each of the secondary windings 50 and 52 is of a conventional design, and, as such, is wound about its own annular magnetic core 54, which encompasses the pri mary conductor 40. The turns of each of these windings are electrically insulated from each other and from the core in a conventional manner. The cores 54 are at ground potential, as is one point on each of the secondary windings. The space located internally of the magnetic core 54 is referred to herein as the window of the current transformer secondary.

These secondary windings 50 and 52 are enclosed by grounded metal shields 60 that serves to electrostatically shield the windings by preventing concentrations of electric stress adjacent the irregular surfaces thereof. In this connection, the metallic shield 60 is of a generally toroidal form with a smoothly curved external surface to reduce stress concentrations thereadjacent. Preferably shield 60 is made of two generally semi-toroidal components, each of which is suitably attached to the fixture 22. Suitable insulation 59 is provided between the two parts of the shield 60 to prevent the shield from forming a shortcircuited turn around the cores 54.

Shield 60 and windings 50 and 52 are at substantially ground potential, but suitable electrical insulation 64 is provided between them to maintain electrical isolation between the secondary winding circuits and the conductive shield 60. The secondary windings 50 and 52 and shield 60 constitute a unitary assembly that is suitably attached to the fixture 22 and is thus supported on fixture 22 and tubular insulator 24.

In the illustrated embodiment, the means for attaching the windings 50, 52 and the shield 60 to the fixture 22 comprises a U-shaped saddle 61 which extends circumferentially of the cores 54 over a portion only of their circumference. The windings 50 and 52 are suitably attached to this saddle 61. Saddle 61 has laterally-projecting lugs 62 welded thereto. Screws 63 are threaded into these lugs, and when tightened, clamp the saddle 61, the shield 60, and the fixture 22 together.

After these parts 50, 52, and 6063 are mounted on the fixture 22, the top portion 14:: of the tank (without stud 40, 42) is mounted on the lower portion 14b, after which the stud 40, 42 is inserted from the left through the wall of tank 14 and through the window of the windings 50, 52 and is suitably secured in place at its opposite ends. This essentially completes the assembly operation.

The tension member 18 is a hollow member that forms 4 a conduit through which the leads of the secondary windings extend. These leads, which are covered by suitable insulation, are shown at 66 extending into the top of the hollow tension member 18 and emerging therefrom at its lower end. They are carried to utilization points through a suitable cover 67 at the lowermost end of the current transformer.

For insulating the grounded structure 60, 18, and 22 from the primary conductor 40 and the tank 14, both of which are at high voltage, a suitable gaseous insulation is provided within tank 14 and hollow insulating column 12. This gaseous insulation is preferably sulphur hexafiuoride at a pressure of several atmospheres. Since the grounded structure 60, 18, 22 is spaced from the high voltage parts, the gaseous insulation provides the necessary dielectric strength between these parts to normally prevent electric breakdowns, or flashovers, within the current transformer assembly. Pressure-tight joints throughout the assembly prevent any significant leakage of the pressurized gas from the assembly. Although I prefer to use a pressurized gas as the insulating medium, the invention in its broader aspects is also applicable to liquid-filled current transformer assemblies.

For further modifying the electrostatic field distribution to prevent undesirable stress concentrations, additional electrostatic shields 70 and 72 are provided. Shield 70 is a tubular metal member surrounding tension mem ber 18 in spaced relation thereto at the top of the insulating column 12. This shield is supported from and electrically connected to tank 14. The other shield 72 is a cup-shaped member surrounding stud 30 and nuts 32 at the lower end of the assembly. This shield, which is at ground potential, is preferably supported from and electrically connected to the lower flange 34.

-As pointed out hereinabove, the insulating column 12 of the current transformer must be capable of withstanding large mechanical forces, as might result, for example, from high pressures inside the assembly, from high winds, or from heavy ice loads applied to cable 41 attached to stud 40. These high forces will tend to load the insulating column '12 in tension; and this is a prob lem because porcelain, which is the usual insulating ma terial used for column 12, is relatively weak in tension. To increase its ability to safely withstand high loads, it has been proposed that means be provided for preloading the insulating column in compression.

Prior arrangements for compressively preloading the insulating support column of current transformers have usually comprised a series of spaced apart insulating rods extending through the column between its opposite ends. Not only is such an arrangement expensive and complicated, but it consumes so much space that it is usually necessary to increase the size of the column 12 to accommodate the rods and still provide the necessary electrical clearances.

I am able to eliminate need for such insulating rods inasmuch as I rely upon the central tubular member 18 as a tension member for exerting compressive force on porcelain insulating column 12. This member 18 is of metal instead of insulating material and, moreover, can be of a high strength and inexpensive metal, such as steel, since it is not required to conduct any current. This permits high forces to be transmitted through the tension member despite its relatively small cross section.

Even in the absence of a tension member, such as 18, a suitable conduit would be required to carry the leads 66 of the secondary windings to the bottom of column 12. Since the tension member 18 is hollow, it is available to perform this function as well as its force-transmitting function. Accordingly, member 18 is able to perform its basic force-transmitting function without consuming much more space than would have been required by a member used only as a conduit. Since the tension member 18 is of a small cross-section, the surrounding insulating column 12 can likewise have a small diameter and still provide the necessary electrical clearances.

Because the tension member 18 is not required to carry the primary current through the transformer, its diameter or length is not appreciably affected by the magnitude of the primary current. A current-carrying member, on the other hand, would expand and contract in response to the varying heating effect of varying currents. Maintaining a substantially constant length for member 18 is an advantage inasmuch as it permits the spring washers 36 to be of a more simple construction and to exert a more uniform compressive force on the insulating column 12 despite the magnitude of the primary current. Had the tension member 18 lengthened by an appreciable amount in response to high currents, the force exerted by the spring washers 36 would appreciably drop, necessitating some form of compensation.

It should also be noted that since tension member 18 does not carry current, its cross section can be determined independently of its current-carrying ability. If, on the other hand, its cross-section was determined by current-carrying ability, then a large cross-section would have to be used for high currents, in some cases necessitating a larger insulating enclosure. Since the crosssection of my tension member 18 can be determined independently of its current carrying ability, I am able to use a small diameter tension member even for current transformers that must carry very high primary currents.

It should be noted that the tubular insulator 24 plays an important role in maintaining the column 12 under compression since the forces transmitted from the tension member 18 to the insulating column 12 are transmitted through the insulator 24. Insulator 24 serves also to maintain constant gap lengths between the high voltage parts and the grounded parts of the assembly de spite high mechanical forces to which any of these parts might be subjected. In this respect, insulator 24 fixes the upper end of tension member 18 against radial shifting relative to tank 14 and substantially fixes the secondary assembly 50, 52, 60 against shifting radially or axially of the primary conductor 40. In addition, the insulator 24 serves as the support on which the secondary assembly 50, 52, 60 is mounted. There is no need to introduce any other insulators for performing this support function, thus eliminating the expense of any such additional insulators and avoiding additional potential breakdown paths inside tank '14.

Although the live tank type of current transformer described hereinabove is a preferred form of my invention, the invention in its broader aspects also has application to the grounded tank type of current transformer.

While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects; and I, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A high voltage current transformer comprising: (a) a hollow column primarily of insulating material, (b) a metal tank mounted atop said column and having an opening therein that is generally aligned with the interior of said column, (c) a high voltage primary conductor extending through said tank and electrically connected thereto, (d) a low voltage secondary winding assembly disposed within said tank and surrounding said primary conductor in spaced relationship thereto,

(e) a tubular insulator within said tank generally aligned with said column,

(f) means for mounting said secondary winding atop said tubular insulator,

(g) a tubular metal tension member extending through said insulator and said column,

(h) force-transmitting means between the upper end of said tension member and the upper end of said tubular insulator for applying compressive force to said tubular insulator and said column when said tension member is subjected to tensile forces,

(i) means for applying a tensile force to said tension member,

(j) and conductive leads from said secondary winding extending through said tubular metal member to a region adjacent the lower end of said column.

2. The current transformer of claim 1 in which said means for applying a tensile force to said tension member comprises spring means near the lower end of said column exerting a downward force on said tension member.

3. The current transformer of claim 2 in combination with adjustable means adjacent the upper end of said tension member for varying the force exerted by said spring means, said adjustable means being operable to vary the effective length of said tension member.

4. The current transformer of claim 1 in which said tank and said insulating column are filled with gaseous insulation under a pressure substantially exceeding atmospheric pressure."

5. A high voltage current transformer comprising:

(a) a hollow column primarily of insulating material,

(b) a metal tank fixed to said hollow column at one end thereof and having an opening therein that is generally aligned with the interior of said column,

(c) a high voltage primary conductor extending into said tank,

((1) a low voltage secondary winding assembly disposed within said tank and surrounding said primary conductor in spaced relationship thereto,

(e) a tubular insulator within said tank generally aligned with said column,

(f) a metal tension member extending through said insulator and said column,

(g) force-transmitting means located within said tank between one end of said tension member and one end of said tubular insulator for applying compressive force to said tubular insulator and said column when said tension member is subjected to tensile force,

(h) and means for applying a tensile force to said tension member.

6. The current transformer of claim 5 in which said means for applying a tensile force to said tension member comprises spring means at the opposite end of said tension member from said force-transmitting means.

7. The current transformer of claim 6 in combination with adjustable means adjacent said one end of said tension member for varying the force exerted by said spring means, said adjustable means being operable to vary the effective length of said tension member.

References Cited UNITED STATES PATENTS 10/1943 Camilli 336-473 XR 10/1966 Wilson 336-l74 XR 

